This invention relates to a laminated glass incorporating therein a hologram sheet. For example, the laminated glass panel is a vehicle windshield provided with a hologram sheet as a combiner of a head-up display.
It is known to dispose a hologram sheet in a laminated glass panel such as a vehicle windshield. Usually a laminated glass is produced by using a polyvinyl butyral (PVB) film as an adhesive intermediate film between two glass plates. Accordingly a hologram sheet incorporated in a laminated glass panel makes tight contact with a glass surface on one side and with the PVB film on the opposite side. With this construction the hologram sheet is not always sufficient in endurance to the environmental conditions.
For stronger protection of a hologram sheet incorporated in a laminated glass panel, JP-A 64-44414 shows using two sheets of PVB film as the adhesive and sandwiching the hologram sheet between the two PVB films. By this measure the hologram sheet is protected very firmly. However, with this construction it is often that in reproducing a holographic diffraction image the diffraction wavelength of the hologram sheet significantly deviates from the aimed wavelength. The amount of the deviation depends on the kind of the PVB film used in the laminated glass and in some cases reaches 80-100 nm so that the color of the holographic image differs from the intended color. For use in laminated glass there are many kinds of PVB films on the market, and it is inconvenient and difficult to exactly choose a PVB film which does not greatly affect the diffraction wavelength of a hologram sheet brought into contact with the PVB film.
Besides, when a hologram sheet is sandwiched between two sheets of PVB film in a laminated glass panel it is often that the holographic diffraction images are blurred.